Search for the decay $B^+ \rightarrow K^+\tau^+\tau^-$ using data from the Belle and Belle II experiments

TL;DR

This paper searches for the rare decay of B+ mesons into K+ and tau pairs using data from Belle and Belle II, setting a new upper limit on its branching fraction after no significant signal was observed.

Contribution

First search for B+ to K+ tau+ tau- decay using combined Belle and Belle II data, improving the upper limit by a factor of four.

Findings

No significant excess observed.

Set an upper limit on branching fraction: < 0.56 x 10^-3.

Improved previous limit by a factor of four.

Abstract

We report a search for the rare decay $B^{+} \rightarrow K^{+} \tau^{+} \tau^{-}$ using $1.2 \times 10^9$ $\Upsilon(4S)$ mesons produced near threshold in electron-positron collisions and collected by the Belle and Belle~II experiments. We fully reconstruct the hadronic decay of one $B$ meson produced in the $\Upsilon(4S)\rightarrow B^{+} B^{-}$ decay, and search for $B^{\pm}\rightarrow K^{\pm} \tau^{+}\tau^{-}$ candidates among the remaining collision products, reconstructing a charged kaon and leptonic decays of the $\tau$ leptons. We optimize the selection for best sensitivity and look for an excess over background at low values of the residual energy detected in the calorimeter after full event reconstruction. We observe no significant excess and set the limit $\mathcal{B}(B^{+}\rightarrow K^{+}\tau^{+}\tau^{-})< 0.56\times 10^{-3}$ at the 90% confidence level, improving on the only previous result by a factor of four.